Teaching in the black in Spain

I am considering teaching in Spain. I am aware, from my time in Slovakia, that the original EU member nations are difficult to get work visas for unless you have EU papers, which I do not.

I have read that there is little difficultly in getting work in Spain despite this. Without putting anyone in jeopardy of deportation, or flogging or whatever, I wonder if there are people who have personal knowledge of the real possibilities and risks associated with teaching sans visa in Spain. Any information would be helpful. I can be contacted by personal message as well should anyone feel hesitant about posting on the public board.